A practical method involving digital signal accumulation has been developed for determining and characterising the very weak fluorescences (phi approximately 10 to the minus 4th power less than 10 to the minus 5th power) of all the (1971). Early results showed solvent effects in the emission spectra and unexpected excitation spectra which have been proposed to be the first experimental evidence for the tautomeric forms of the bases. (Proc. Nat. Acad. Sci. (in press)). It is proposed first to improve the data-collection and data-handling procedures, and then to use them to investigate in detail the following topics: (a) tautomerism (excitation spectra at increased resolution, solvent and temperature effects on phi f), (b) solvent effects on the excited state parameters of the bases, possibly leading to dipole moments of the excited states, (c) a systematic study of the room temperature excited state behavior of oligo- and poly-nucleotides, (d) the excited state parameters of nucleic acids, particularly with respect to base composition and sequence (where known) and secondary and tertiary structure.